Methods and apparatus for image synthesis

Abstract: According to the method of synthesizing a image from a plurality of images, each of the luminance I p1xy -I pNxy of each of the pixel p 1xy -p Nxy positioned at the same coordinates (x,y) of N images P₁-P N are compared with each of the predetermined threshold luminance T₁-T N corresponding to images P₁-P N , wherein "N" is a natural number greater than 1. If the luminance I p1xy -I pNxy of the pixels p 1xy -p Nxy are all greater than the corresponding threshold T₁-T N of corresponding image, the synthesized pixel p xy at the coordinate (x,y) of a synthesized image is determined by synthesizing the pixels p 1xy -p Nxy with predetermined synthesis ratio for each of the images P₁-P N . If the condition described above is not satisfied, the synthesized pixel p xy at the coordinate (x,y) of the synthesized image is determined by synthesizing with arbitrary synthesis ratio only the pixels having luminance less than or equal to the corresponding threshold T₁-T N among the pixels p 1xy -p Nxy . The procedure described above is performed with regard to all pixels in the region wherein images are to be synthesized.

Description: Background of the Invention Field of the Invention The present invention concerns methods and apparatus for the synthesis of images to be utilized on the terminals of video communication systems, especially those methods and apparatus which are used for the sending and receiving through communication lines of multiple images of writings or hand movements on a desktop of multiple users separated into remote locations. The images are synthesized, and with the use of display apparatus, it is possible for user to point with one's finger or pen and to write words at the desired location in the writings on their respective desktops. Such apparatus would make it possible to perform group work such as office work, design work, or software design on the individual desktops of the respective users. Related Technology Up until now, there has been a video communication terminal which is able to send and receive through communication lines video images of hand movements on the desktops of multiple users who are stationed at separate locations. The terminal offers a shared drawing space in which each person's video image is synthesized translucently, so that it is possible to see through. Displayed on a monitor, it is possible for many people to simultaneously give directions for the desired location on the video image by using a finger or a pen, as well as writing in words (see Japanese Patent Application Laid-Open Publication No. 4-119,087). Fig. 24 shows a block diagram of the structure of a video communication system which utilizes this kind of video communication terminal. In Fig. 24, the two video communication terminals 1a and 1b are able to communicate in two directions through a communication line 2. The video communication terminals 1a and 1b are each made up of video image input apparatuses 3a and 3b, video image synthesis apparatuses 4a and 4b, display apparatuses 5a and 5b, and video image communication apparatuses 6a and 6b respectively. The video image input apparatuses 3a and 3b comprise video cameras, which take image of each person's desktop, and, for example, output the video images 7a and 7b shown in Fig. 25. The video image synthesis apparatus 4a carries out a translucent synthesis, so that the images are able to be seen through, of the video image 7a received directly from the video image input apparatus 3a and the video image 7b, which is received through the video image communication apparatus 6a and the communication line 2 from the video image communication apparatus 6b, which in turn received the image from the video image input apparatus 3b. In this way, the display apparatus 5a receives video image 8 as seen in Fig. 25. Similarly, the video image synthesis apparatus 4b carries out a translucent synthesis, of the video image 7b received directly from the video image input apparatus 3b and the video image 7a, which is received through the video image communication apparatus 6b and the communication line 2 from the video image communication apparatus 6a, which in turn received the image from the video image input apparatus 3a. In this way, the display apparatus 5b receives the video image 8 as seen in Fig. 25. Next, the image synthesis method used by the image synthesis apparatuses 4a and 4b of the video communication system mentioned above (designated Prior Image Synthesis Method 1 from here on) will be explained. Regarding Prior Image Synthesis Method 1, suppose a synthesized image P is created by synthesizing two images P1 and P2, each of which have (X×Y) (X and Y are natural numbers) pixels, at arbitrary synthesis ratios. Suppose the pixels which lie on the coordinates (x,y)(1≦x≦X, 1≦y≦Y, x and y are natural numbers) on each image P1, P2, and P are designated p 1xy , p 2xy , and p xy respectively. If the synthesis ratio between the image P1 and P2 is set at w 1 :w 2 (w 1 +w 2 =1, w 1 >0, w 2 >0), the image synthesis is carried out by using equation (1) on the pixels which are positioned at (x,y) of each image. p xy =w 1 p 1xy +w 2 p 2xy Prior Image Synthesis Method 1 is explained using the flow chart in Fig. 26 as a reference. First. in step SA1, the register y to vary the y coordinates is set at an initial value of "1"; similarly, in step SA2, the register x to vary the x coordinates is set at an initial value of "1". Next, in step SA3, after carrying out the image synthesis using equation (1) for the set x and y coordinates, the procedure advances to step SA4, in which "1" is added to the value of the register x. In step SA5, it is determined whether or not the value of register x is greater than the maximum value X, and if the result is "NO", then the procedure returns to step SA3. Then, the procedure in steps SA3 and SA4 is repeated until the value of the register x becomes greater than the maximum value X. When the value of the register x becomes greater than the maximum X, the result of step SA5 becomes "YES", and the procedure advances to step SA6. In step SA6, "1" is added to the value of the register y. In step SA7, it is determined whether or not the value of the register y is greater than the maximum value Y, and if the result is "NO", then the procedure returns to step SA2. Then, the procedure in steps SA2-SA6 is repeated until the value of the register y becomes greater than the maximum value Y. When the value of the register y becomes greater than the maximum Y, the result of step SA7 becomes "YES", and the procedure ends. In Fig. 27, the result in the case when Prior Image Synthesis Method 1 is used to create a translucent synthesized image is shown symbolically. In Fig. 27, if the synthesis ratio of image 9a and 9b is 0.5:0.5, then as shown in image 10, the synthesis of a black pixel and a white pixel (white pixel = 1, black pixel = 0) becomes a gray (0.5) pixel. Because of this, when synthesizing two video images of desktops using Prior Image Synthesis Method 1; as seen in Fig. 25, the contrast of the drawings of both sides drops, and there was often a problem in seeing document and drawings clearly. In addition, as shown in Fig. 24, the two image communication terminals 1a and 1b are connected through the communication line 2 and each one synthesizes two images, but when the number of images to be synthesized increases due to the addition of more video communication terminals. the contrast decreases; and therefore, regarding the aforementioned Prior Image Synthesis Method 1, it was difficult to put into practice a video communication system which could synthesize several video images by connecting several separate locations through communication lines. Additionally, regarding the aforementioned Prior Image Synthesis Method 1, when sharing document by means of a video image, for example, if a low-volume digital line such as N-ISDN is used for the communication line 2, then upon compression or decompression of the video image the quality drops sharply, and often, when used with business document, small words would become illegible. In order to solve the problems in the aforementioned Prior Image Synthesis Method 1, there is another image synthesis method (designated Prior Image Synthesis Method 2) which adds the following technology to Prior Image Synthesis Method 1. This Prior Image Synthesis Method 2 has technology which adds a apparatus which preserves the document by sending it as high-precision still images, then translucently synthesizes these high-precision still images with the video images from each video communication terminal (see Japanese Patent Application Laid-Open Publication No. 6-153,192). Fig. 28 shows a block diagram of a structural example of a video communication system which uses Prior Image Synthesis Method 2. In Fig. 28, the two video communication terminals 9a and 9b are connected by a communication line 10 and are able to communicate in either direction. The video communication terminals 9a and 9b are made up of video image input apparatuses 11a and 11b, still image memories 12a and 12b, image synthesis apparatuses 13a and 13b, display apparatuses 14a and 14b, and video/still image communication apparatuses 15a and 15b respectively. The video image input apparatuses 11a and 11b comprise high-resolution cameras which are able to capture a document very precisely. Next, the Prior Image Synthesis Method 2 is explained with reference to Fig. 28. First, in the video communication terminal 9a, in order to use the document in both video communication apparatus 9a and 9b, the document which is on a desktop is captured by the video image input apparatus 11a, then a single frame of the captured image is stored in the still image memory 12a as a still image. Next, the video/still image communication apparatus 15a sends the still image, which was stored in the still image memory 12a, through the communication line 10 to the video communication terminal 9b. The video communication terminal 9b stores the still image, which was received by the video/still image communication apparatus 15b from the video communication terminal 9a, in the still image memory 12b. Next, in the video communication terminal 9a, the desktop is captured by the video image input apparatus 11a, the captured video image is sent to the video communication terminal 9b through the communication line 10 by the video/still image communication apparatus 15a. Meanwhile, the still image which was held in the still image memory 12a, the video image which was captured by the video image input apparatus 11a, and the video image from the video communication terminal 9b, which was obtained through the video/still image communication apparatus 15a, are combined translucently. so that each image is able to be seen, by the image synthesis apparatus 13a, and displayed on the display apparatus 14a. Similarly, in the video communication terminal 9b, the desktop is captured by the video image input apparatus 11b, the captured video image is sent to the video communication terminal 9a through the communication line 10 by the video/still image communication apparatus 15b. Meanwhile, the still image which was held in the still image memory 12b, the video image which was captured by the video image input apparatus 11b, and the video image from the video communication terminal 9a, which was obtained through the video/still image communication apparatus 15b, are combined translucently, so that each image is able to be seen, by the image synthesis apparatus 13b, and displayed on the display apparatus 14. In the aforementioned video communication terminals 9a and 9b, as shown in Fig. 29, when the video image 16a (captured by the video image input apparatus 11a) taken by capturing one's own desktop, video image 16b (captured by the video image input apparatus 11b) taken be capturing another person's desktop, and the still image 17 which was held in the still image memory 12a are synthesized translucently so that each image is visible. as shown in the image 18, it is possible to create a shared work space in which both video communication apparatus 9a and 9b can simultaneously write in comments regarding the still image 17. In the aforementioned Prior Image Synthesis Method 2, by sending document as still images separate from the video images and making the document image high precision, the need to write directly on the paper is dropped, and it is possible for both sides to write on the document image simultaneously. However, in the aforementioned Prior Image Synthesis Method 2, because the still image is combined with the video images translucently, as shown in the synthesized image 18 in Fig. 29, the contrast of the still image drops, and a problem with not being able to read the document remains. EP-0 335 036 describes an image composing apparatus adapted to produce a composite image in which a value of each of picture elements is represented by a maximum value or minimum value of the brightness of a video signal as well as a corresponding other signal at the instant of the maximum or minimum value. A composite image is formed in which a value of each of picture elements is represented by a count of synchronizing signals or an externally applied timing, position or angle indicative external input signal at an instant that the video signal attains the maximum value or the minimum value for each picture element. Summary of the Invention The object of this invention is to offer methods and apparatus for image synthesis which can eliminate the visual problems caused by the drop in contrast resulting when multiple video images and still images are synthesized translucently, and make it possible for a video communication terminal to offer a shared work space connecting several locations. Therefore, this invention compares the luminances I p1xy -I pNxy of each pixel p 1xy -p Nxy positioned at the same coordinates (x,y) of N (N is a natural number greater than 1) images P 1 -P N , and determine whether all of the luminances I p1xy -I pNxy are greater than their threshold values of T 1 -T N . If all of those luminances are greater, then the respective pixels p 1xy -p Nxy of each image P 1 -P N are synthesized at their predetermined synthesis ratios, and the result of the syntheses are taken as the synthesized pixels p xy at the relevant coordinates (x,y) of the synthesized image. Otherwise, of the respective pixels p 1xy -p Nxy of the images P 1 -P N , only the pixels which have a luminance I p1xy -I pNxy less than the threshold values T 1 -T N are synthesized at the arbitrary synthesis ratios, and the results of the synthesis for each coordinate (x,y) on the synthesized image is taken as the synthesized pixel p xy . The procedure is carried out for each region which needs to be synthesized. In addition, the invention is an image synthesis method which synthesizes N (N is a natural number greater than 1) video images V 1 -V N and M (M is a natural number) still images S 1 -S M . If M is equal to "1", then the relevant still image S 1 is taken as the synthesized still image S, and if M is greater than "1", then it compares the respective luminance I s1xy -I sMxy of those pixels s 1xy -s Nxy lying on the same coordinates (x,y) of the M still images with the respective threshold luminance T s1 -T sM which have been predetermined for each previous still image S 1 -S M to see whether all of the luminances I s1xy -I sMxy are greater than their respective thresholds T s1 -T sM . If all of those luminances are greater, then the respective pixels s 1xy -s Mxy of the previous still images S 1 -S M are synthesized at the predetermined synthesis ratios, and the result of the synthesis is taken as synthesized still image pixel s xy on the relevant coordinates (x,y) of synthesized still image S. Otherwise, of the respective pixels s 1xy -s Mxy of the previous still image S 1 -S M , only those which have luminances I s1xy -I sMxy less than or equal to the threshold values T 1 -T M are synthesized at their respective predetermined synthesis ratios, and the result of the synthesis is taken as synthesized still image pixel s xy on the relevant coordinates (x,y) of synthesized still image S. This procedure is repeated for all regions which need to be synthesized in order to create synthesized still image S. Then it compares the respective luminance I v1xy -I vNxy of those pixels v 1xy -v Nxy lying on the same coordinates (x,y) of the N video images with the respective threshold luminance T v1 -T sN which have been predetermined for each previously mentioned video image V 1 -V N to see whether all of the luminances I v1xy -I vNxy are greater than their respective thresholds T v1 -T vN . If all of those luminances are greater, then the synthesized still image pixel s xy positioned at the relevant coordinates (x,y) on the previously mentioned synthesized still image S is taken as the synthesized pixel p xy located at the relevant coordinates (x,y) of synthesized image P. Otherwise, then of the respective pixels v 1xy -v Nxy of the previously mentioned video images V 1 -V N , only those pixels with luminances I v1xy -I vNxy which are less than or equal to their threshold values T 1 -T N are synthesized at their arbitrary synthesis ratios, and the result of the synthesis is taken as synthesized pixel p xy located at the relevant coordinates (x,y) on synthesized image P. This procedure is repeated for all regions which need to be synthesized. In addition, the invention is an image synthesis method which synthesizes N (N is a natural number greater than 1) video images V 1 -V N and M (M is a natural number) still images S 1 -S M . If M is equal to "1", then the relevant still image S 1 is taken as the synthesized still image S, and if M is greater than "1", then it compares the respective luminance I s1xy -I sMxy of those pixels s 1xy -s Mxy lying on the same coordinates (x,y) of the M still images, and takes the pixel with the least luminance as the synthesized still image pixel s xy positioned at the relevant coordinates (x,y) on synthesized still image S. This procedure is repeated over all regions which need to be synthesized and the result is the synthesized still image S. Then it compares the respective luminance I v1xy -I vNxy of those pixels v 1xy -v Nxy lying on the same coordinates (x,y) of the N video images V 1 -V N with the respective threshold luminance T v1 -T vN which have been predetermined for each previously mentioned video image V 1 -V N to see whether all of the luminances I v1xy -I vNxy are greater than their respective thresholds T v1 -T vN . If they are greater, then the synthesized still image pixel s xy positioned at the relevant coordinates (x,y) on the previously mentioned synthesized still image S is taken as the synthesized pixel p xy located at the relevant coordinates (x,y) of synthesized image P. Otherwise, of the respective pixels v 1xy -v Nxy of the previously mentioned video images V 1 -V N , the pixel with the lowest luminance out of those pixels with luminances I v1xy -I vNxy less than or equal to their thresholds T v1 -T vN is taken as the synthesized pixel p xy located at the relevant coordinates (x,y) on the synthesized image P. The procedure is repeated over all regions which need to be synthesized. Additionally, the invention is an image synthesis method which synthesizes N (N is a natural number greater than 1) video images V 1 -V N and M (M is a natural number) still images S 1 -S M . If M is equal to "1", then the relevant still image S 1 is taken as the synthesized still image S, and if M is greater than "1", then it compares the respective luminance I s1xy -I sMxy of those pixels s 1xy -s Mxy lying on the same coordinates (x,y) of the M still images, and takes the pixel with the least luminance as the synthesized still image pixel s xy positioned at the relevant coordinates (x,y) on synthesized still image S. This procedure is repeated over all regions which need to be synthesized and the result is the synthesized still image S. Then it compares the respective luminance I v1xy -I vNxy of those pixels v 1xy -v Nxy lying on the same coordinates (x,y) of the N video images V 1 -V N and chooses the pixel with the lowest luminance. If the luminance of the pixel is greater than the predetermined threshold value, then the synthesized still image pixel s xy which lies on the relevant coordinates (x,y) of the previously mentioned synthesized still image S is taken as the synthesized pixel p xy lying on the relevant coordinates (x,y) of the synthesized image P. Otherwise, then the previously mentioned pixel with the lowest luminance is taken as synthesized pixel p xy lying on the relevant coordinates (x,y) of synthesized image P. This procedure is repeated for all regions which need to be synthesized. Additionally, the invention is an apparatus for synthesizing an image from a plurality of N input images P1-PN, comprising: comparing means for comparing the luminance I p1xy- I pNxy of each pixel P 1xy- P Nxy positioned at the respective same coordinates x, y of each input image P 1 -P N with a predetermined threshold value T 1- T N which is respectively given for each input image; synthesizing means for determining the luminance of a synthesized pixel p xy at said coordinate x, y of a synthesized image by synthesizing said pixels p 1xy -p Nxy with a predetermined synthesis ratio given for each of said input images P 1 -P N , if the luminance I p1xy -I pNxy of all said pixels p 1xy -p Nxy is greater than the corresponding threshold T 1 -T N , or determining the luminance of said synthesized pixel p xy at said coordinate x, y by synthesizing only those pixels whose luminance is less than or equal to the corresponding threshold T 1 -T N among said pixels p 1xy -p Nxy , if the condition described above is not satisfied; and means for carrying out the above procedure for each image region which needs to be synthesized. Additionally the present invention is an apparatus for synthesizing an image from N video images and M still images, N being a natural number greater than 1 and M being a natural number, comprising: means for taking the sole still image S1 as a synthesized still image S if M is 1; first comparison means for comparing the luminance I s1xy -I sMxy of each pixel S 1xy -S Mxy positioned at the respective same coordinates x, y of the M still images S 1 -S M with a predetermined threshold value T s1 -T sM which is respectively given for each still image, if M is greater than 1; first synthesizing means for determining a synthesized still pixel s xy at said coordinate x, y of a synthesized still image S; means for carrying out the above procedure for each image region which needs to be synthesized; second comparison means for comparing the luminance I v1xy -I vNxy of each pixel v 1xy -v Nxy positioned at the respective same coordinates x, y of the N video images V 1 -V N with a predetermined threshold value T v1 -T vN which is respectively given for each video image; second synthesizing means for taking said synthesized still pixel s xy at said coordinate x, y of said synthesized still image S as a synthesized pixel p xy at said coordinate x, y of a synthesized image P, if the luminance I v1xy -I vNxy of all said pixels v 1xy -v Nxy is greater than the corresponding threshold T v1 -T vN , or determining the luminance of said synthesized pixel p xy at said coordinate x, y of said synthesized image P among those pixels v 1xy -v Nxy which have a luminance less than or equal to the corresponding threshold T v1 -T vN , if the condition described above is not satisfied; and means for carrying out the above procedure for each image region which needs to be synthesized. Additionally the present invention is an apparatus wherein said first comparison means outputs first information if said luminance I s1xy -I sMxy are all greater than the corresponding threshold value T s1 -T sM , or outputs second information regarding the luminance signals less than or equal to the corresponding threshold value T s1 -T sM if the condition described above is not satisfied; said first synthesizing means synthesizes said synthesized still image from said still images S 1 -S M with predetermined synthesis ratio for each of said still images S 1 -S M if said first information is inputted, or synthesizes said synthesized still image from said corresponding images to said second information with arbitrary ratio if said second information are inputted; said second comparison means outputs third information indicating that the corresponding condition is satisfied if said luminance I v1xy -I vNxy are all greater than the corresponding threshold luminance T v1 -T vN , or outputs fourth information regarding the luminance signals less than or equal to the corresponding threshold luminance T v1 -T vN if the condition described above is not satisfied; and said second synthesizing means outputs said synthesized still image as a synthesized image if said third information is inputted, and outputs synthesized image from image having the corresponding luminance signals to said fourth information among said images V 1 -V N with arbitrary ratio if the condition described above is not satisfied. With the use of the present invention the clarity of text and drawings in shared work spaces increases: especially when using document as a still image. the drop in contrast is able to be eliminated. In addition, especially in the case when several video images are synthesized, since a similar increase in the contrast is possible, there is the advantage that a multiple-location shared work space can be created by linking several points and synthesizing several desktop images. There is also the advantage that a shared work space is possible in which it is not necessary to write directly on the text in the still image. Brief Description of the Drawings Fig. 1 is a flow chart showing the image synthesis method of Embodiment 1 of the present invention. Fig. 2 is a flow chart to show the procedure in the case in which two images P 1 and P 2 are synthesized using Embodiment 1 of the present invention. Fig. 3 is a diagram showing the way in which two images P 1 and P 2 are synthesized using Embodiment 1 of the present invention. Fig. 4 is a flow chart to explain the image synthesis method of Embodiment 2 of the present invention. Fig. 5A is a flow chart to explain the image synthesis method of Embodiment 3 of the present invention. Fig. 5B is a flow chart to explain the image synthesis method of Embodiment 3 of the present invention. Fig. 5C is a flow chart to explain the image synthesis method of Embodiment 3 of the present invention. Fig. 5D is a flow chart to explain the image synthesis method of Embodiment 3 of the present invention. Fig. 5E is a flow chart to explain the image synthesis method of Embodiment 3 of the present invention. Fig. 6 is a flow chart showing the procedure in the case in which two video images V 1 and V 2 and a single still image S are synthesized using Embodiment 3 of the present invention. Fig. 7A is a flow chart showing the procedure in the case in which N video images V 1 -V N and a single still image S are synthesized using Embodiment 3 of the present invention. Fig. 7B is a flow chart showing the procedure in the case in which N video images V 1 -V N and a single still image S are synthesized using Embodiment 3 of the present invention. Fig. 7C is a flow chart showing the procedure in the case in which N video images V 1 -V N and a single still image S are synthesized using Embodiment 3 of the present invention. Fig. 7D is a flow chart showing the procedure in the case in which N video images V 1 -V N and a single still image S are synthesized using Embodiment 3 of the present invention. Fig. 8A is a flow chart to explain the image synthesis method of Embodiment 4 of the present invention. Fig. 8B is a flow chart to explain the Image synthesis method of Embodiment 4 of the present invention. Fig. 9 is a flow chart to explain the image synthesis method of Embodiment 5 of the present invention. Fig. 10 is a block diagram to explain the image synthesis apparatus of Embodiment 6 of the present invention. Fig. 11 is a block diagram to explain the image synthesis apparatus of Embodiment 7 of the present invention. Fig. 12 is a block diagram showing a structural example of an image communication system making use of the image synthesis apparatus described in Embodiment 7 of the present invention. Fig. 13 is a diagram to explain the synthesis of the desktop images of each image communication terminal 41a and 41b using the image communication system shown in Fig. 12. Fig. 14 is a block diagram showing the structure of the image synthesis apparatus described in Embodiment 8 of the present invention. Fig. 15 is a block diagram showing the structure of the image synthesis apparatus which synthesizes two images P 1 and P 2 , described in Embodiment 8 of the present invention. Fig. 16 is a block diagram showing a structural example of an image communication system making use of the image synthesis apparatus shown in Fig. 15. Fig. 17 is a diagram to explain the synthesis of the desktop images of each image communication terminal 62a and 62b using the image communication system shown in diagram 16. Fig. 18 is a block diagram showing the structure of the image synthesis apparatus described in Embodiment 9 of the present invention. Fig. 19 is a block diagram showing the structure of the image synthesis apparatus described in Embodiment 10 of the present invention. Fig. 20 is a block diagram showing a structural example of an image communication system making use of the image synthesis apparatus described in Embodiment 10 of the present invention. Fig. 21 is a diagram to explain the synthesis of the desktop images of each image communication terminal 101a and 101b using the image communication system shown in diagram 20. Fig. 22 is a block diagram showing the structure of the image synthesis apparatus described in Embodiment 11 of the present invention. Fig. 23 is a block diagram showing the structure of the image synthesis apparatus described in Embodiment 12 of the present invention. Fig. 24 is a block diagram showing a structural example of an image communication system using Prior Image Synthesis Method 1. Fig. 25 is a diagram to explain Prior Image Synthesis Method 1. Fig. 26 is a flow chart to explain Prior Image Synthesis Method 1. Fig. 27 is a diagram showing symbolically how a translucent synthesized image is created by Prior Image Synthesis Method 1. Fig. 28 is a block diagram showing a structural example of an image communication system using Prior Image Synthesis Method 2. Fig. 29 is a diagram to explain Prior Image Synthesis Method 2. Detailed Description of the Invention Below is an explanation of the embodiments of the invention, using the figures as a guide. Embodiment 1 First, an explanation of the invention's first embodiment. Regarding Embodiment 1, N (N is a natural number greater than 1) images P 1 -P N , which are each made up of (X×Y) pixels (X and Y are natural numbers), are mixed in a ratio of W1:W2:...:W N-1 :W N (with the condition that W 1 +W 2 +...+W N-1 +W N =1: W 1 , W 2 , ..., W N-1 , W N >0) and a synthesized image is formed. In this case, the pixel corresponding to each coordinate (x,y) (1≦x≦X, 1≦y≦Y, where x and y are natural numbers) in each image P n (1≦n≦N, n is a natural number) is designated p nxy , and each such pixel p nxy is given a luminance I pnxy (0≦I pnxy ≦1; wherein 0 = black, 1 = white). In addition, for each image P n , the threshold luminance T n (0≦T n ≦1) for each pixel is set. Next, the image synthesis method of the first embodiment of the invention is explained with reference to the flow chart shown in Fig. 1. First, in step SB1, the register for the y coordinates is set to "1" as an initial value, likewise, in step SB2, the register for the x coordinates is set to "1". Next, in step SB3, the initial value of the variable n which indicates the order of the images P is set at "1", the value of the variable c which represents the number of pixels which have a luminance greater than or equal to the threshold T n is set at "0", and the value of w which is the sum of the synthesis ratios of the pixels which have a luminance greater than or equal to the threshold T n is set at "0", then the procedure advances to step SB4. In step SB4, it is determined whether or not the luminance I pnxy of the pixel p nxy which is positioned at the coordinates (x,y) of the relevant image P n is greater than or equal to the threshold T n . If the answer is "YES" then the procedure advances to step SB5. In step SB5, after "1" is added to the variable c and then the procedure advances to step SB6, where the synthesis ratio of the relevant pixel p nxy is added to the sum of the synthesis ratios w, the procedure moves on to step SB7. Additionally, if the result of step SB4 is "NO", that is, if the luminance I pnxy of pixel p nxy is less than the threshold T n , the procedure advances to step SB7 also. In step SB7, "1" is added to the variable n, and in step SB8, it is determined whether or not variable n is greater than the number of images N. If the answer is "NO", then the procedure returns to step SB4. Then, steps SB4-SB7 are repeated using each pixel p nxy which corresponds to the coordinates (x,y) in all of the images. Thus, for the coordinates (x,y) in each image P n , the number c of pixels p nxy which have a luminance I pnxy grater than or equal to the threshold T n and the sum of the synthesis ratios w is calculated. Then, the procedure from step SB4 through SB7, considering each pixel p nxy which lies on the coordinates (x,y) of each image, is completed. The value of variable n becomes greater than N, the result of step SB8 becomes "YES", and the procedure advances to step SB9. In step SB9, the initial value of variable n is set at "1", and the initial value of p which indicates the synthesized value of N pixels p nxy which are positioned on the coordinates (x,y) of N images, is set at "0". In step SB10, it is determined whether variable c is equal to "0" or equal to N, that is, whether the luminances of all N pixels which are positioned on the coordinates (x,y) of N images is less than the threshold T n (c = 0), or else it is greater than or equal to the threshold T n (c = N). If the result is "YES", then the procedure advances to step SB11. In step SB11, the p is constructed with the use of equation (2) and the procedure moves on to step SB12. p = p + w n p nxy In step SB12, "1" is added to variable n, and in step SB13, it is determined whether variable n is greater than the number N of images. If the answer is "NO", then the procedure returns to step SB11. Then, the procedures of SB11 and SB12 are repeated with each pixel p xny positioned at each coordinate (x,y) of all the images. Thus, all of the pixels p nxy which correspond to each coordinate (x,y) in each image at a ratio w n are combined. Then, each pixel p nxy corresponding to the coordinates (x,y) in all of the images completes the procedure in steps SB11 and SB12. and when the value of variable n becomes greater than N, the result of step SB13 becomes "YES", and the procedure advances to step SB19. On the other hand, if the result of step SB10 is "NO", that is, if variable c is equal to neither "0" nor N, then the procedure advances to step SB14. In step SB14, the average value W (= w/c) of the synthesis ratio with regard to the pixels having luminance greater than or equal to the threshold T n is calculated, and in step SB15, it is determined whether the luminance I pnxy of each pixel p nxy positioned on the coordinates (x,y) of the relevant image P n is less than the threshold T n . If the answer is "YES", the procedure advances to step SB16. In step SB16, the p is created by using equation (3) and the procedure moves on to step SB17. p = p + (W + w n )p nxy Of course, if the result of step SB15 is "NO", meaning that the luminance I pnxy of the pixel p nxy is not less than the threshold T n , the procedure also advances to step SB17. In step SB17, "1" is added to the variable n, and in step SB18, it is determined whether variable n is greater than the number N of images. If the answer is "NO", the procedure returns to step SB15. Then, the procedure of steps SB15-SB17 is repeated with each pixel p nxy corresponding to each coordinate (x,y) in the image. Thus, the procedure from step SB15 through step SB17 is completed for each pixel p nxy corresponding to each coordinate (x,y) in the image. If the value of variable n becomes greater than N, the result of step SB18 becomes "YES" and the procedure advances to step SB19. In step SB19, the pixel p xy which corresponds to the coordinates (x,y) of the synthesized image P is set equal to the p and then the procedure advances to step SB20. In step 20, "1" is added to the value of the register x. In step SB21, it is determined whether or not the value of register x is greater than its maximum value of X, and if the answer is "NO", the procedure returns to step SB3. Then, the procedure of steps SB3-SB20 is repeated until the value of the register x becomes greater than its maximum value of X. When the value of the register x becomes greater than the maximum value and the determination of step SB21 becomes "YES", then advance to step SB22. In step SB22, "1" is added to the value of the register y. In step SB23, it is determined whether or not the value of the register y is greater than the maximum value Y, and if the result is "NO", then the procedure returns to step SB2. Then, the procedure of steps SB2-SB22 is repeated until the value of the register y becomes greater than the maximum value of Y. When the value of the register y becomes greater than the maximum value Y, and the result of step SB23 becomes "YES", then the procedure ends. In the aforementioned Embodiment 1, when synthesizing pixels, an example was used in which the synthesis ratios of the non-synthesizing pixels were averaged and distributed (the procedure in step SB16), but by using equation (4), it is possible to distribute the synthesis ratio of the non-synthesizing pixels proportionally with the synthesis ratio of the synthesizing pixels. Next, regarding the aforementioned Embodiment 1, if N = 2, that is, in the case in which there are two images P 1 and P 2 which are to be synthesized, the procedure is explained with the help of the flow chart which is given in Fig. 2. First, in step SC1, the register to vary the y coordinates is set at an initial value of "1", and in step SC2, the initial value of the register to vary the x coordinate is set at "1" as well. Next, in step SC3, it is determined whether or not the luminance I p1xy of the pixel p 1xy positioned at coordinates (x,y) on image P 1 is greater than or equal to the threshold T 1 , and the luminance I p2xy of the pixel p 2xy positioned at coordinates (x,y) on image P 2 is less than the threshold T 2 . If the result is "YES" then the procedure advances to step SC4. In step SC4, the synthesized pixel p xy corresponding to the coordinates (x,y) on the synthesized image P is set equal to pixel p 2xy of image P 2 and then the procedure advances to step SC8. If the result of step SC3 is "NO", that is, if the luminance I p1xy of pixel p 1xy is less than the threshold of T 1 , or if the luminance I p2xy of pixel p 2xy is greater than or equal to the threshold of T2, then the procedure advances to step SC5. In step SC5, it is determined whether or not the luminance I p1xy of pixel p 1xy is less than the threshold T 1 , and the luminance I p2xy of the pixel p 2xy is greater than or equal to the threshold T 2 . If the result is "YES", then the procedure advances to step SC6. In step SC6, the synthesized pixel p xy is set equal to the pixel p 1xy and the procedure advances to step SC8. On the other hand, if the result in step SC5 is "NO", that is, if the luminance I p1xy of pixel p 1xy is greater than or equal to the threshold T 1 , or the luminance I p2xy of pixel p 2xy is less than the threshold T2, then the procedure advances to step SC7. In step SC7, the synthesized pixel p xy is constructed by using equation (5) and then the procedure advances to step SC8. p xy = w 1 p 1xy + w 2 p 2xy In step SC8, "1" is added to the value of the register x. In step SC9, it is determined whether or not the value of the register x is greater than the maximum value X, and if the result is "NO", then the procedure returns to step SC3. Then, the procedure in steps SC3-SC8 is repeated until the register's value is greater than the maximum value X. When the value of the register x becomes greater than the maximum of X, and the result of step SC9 becomes "YES", then the procedure advances to step SC10. In step SC10, "1" is added to the value of the register y. In step SC11, it is determined whether or not the value of the register y is greater than the maximum value of Y, and if the result is "NO", then the procedure returns to step SC2. Then, the procedure in steps SC2-SC10 is repeated until the register's value is greater than the maximum value Y. When the value of the register y becomes greater than the maximum of Y, and the result of step SC11 becomes "YES", then the procedure ends. Fig. 3 shows symbolically how a synthesized image P is constructed out of two images P 1 and P 2 by utilizing the image synthesis method described above in Embodiment 1. In Fig. 3, if image 20a and image 20b are synthesized at a synthesis ratio of 0.5:0.5 following the flow chart in Fig. 2, a synthesized image with a clear contrast can be achieved. In this embodiment, as is clear in contrast to Fig. 27, pixels which are made by synthesizing white pixels with black pixels do not become gray. they become black pixels, and the lessening of contrast can be avoided. The image synthesis method mentioned above in Embodiment 1 may be written as a program, stored in a memory apparatus such as a ROM, and run by a CPU, but it is also possible to build it into the hardware. Embodiment 2 (considered as being not part of invention) Next, the second embodiment is explained. Regarding Embodiment 2, when the N (N is a natural number greater than 1) images P 1 through P N each of which are made up of (X×Y) pixels (X and Y are both natural numbers) are synthesized, the synthesized image P is constructed bY taking the pixel with the lowest luminance among those pixels of the images P 1 through P N which lie on the same coordinates to be the synthesized pixel. In this case, the pixel which lies on the coordinates (x,y) (1≦x≦X, 1≦y≦Y, x and y are natural numbers) on each image P n (1≦nx≦N, n is a natural number) is designated pixel p nxy and the luminance of pixel p nxy is designated I pnxy (0≦I pnxy ≦1 wherein 0 = black, 1 = white). Next, the image synthesis method of the second embodiment is explained with the use of the flow chart in Fig. 4 as a reference. First, in step SD1, the initial value of register y which is used to vary the y coordinates is set at "1", and in step SD2, the initial value of register x which varies the x coordinates is set at "1". Next, in step SD3, the initial value of the variable n which indicates the order of images P is set at "2", the initial value of the lowest luminance 1 is set equal to the luminance I p1xy of pixel p 1xy , and the initial value of p which indicates value of synthesized pixel is set equal to the pixel p 1xy , then the procedure moves on to step SD4. In step SD4, it is determined whether or not the luminance I pnxy of the relevant pixel p nxy is less than luminance i. If the result is "YES", then the procedure advances to step SD5. In step SD5, after the luminance i is set equal to luminance I pnxy , the procedure goes to step SD6. In step SD6, the p is set equal to the pixel p nxy and the procedure advances to step SD7. In step SD7, "1" is added to the variable n, and in step SD8, it is determined whether or not variable n is greater than the number of images N; if the result is "NO", then the procedure goes back to step SD4. Then, the procedure in steps SD4-SD7 is repeated with each pixel p nxy positioned at coordinates (x,y) for all of the images. Then, for each pixel p nxy positioned at coordinates (x,y) for all of the images, the above procedure from steps SD4 through SD7 is completed, the value of variable n becomes greater than N, the result of step SD8 becomes "YES", and the procedure advances to step SD9. In step SD9, the synthesized pixel p xy positioned at coordinates (x,y) on synthesized image P is set equal to the p, then the procedure advances to step SD10. In step SD10, "1" is added to the value of register x. In step SD11, it is determined whether or not the value of register x becomes greater than the maximum value of X, and if the result is "NO", then the procedure returns to step SD3. Then, the procedure from step SD3 through step SD10 is repeated until the value of register x becomes greater than the maximum value X. When register x becomes greater than the maximum of X, and the result in step SD11 becomes "YES", then the procedure moves on to step SD12. In step SD12, "1" is added to the value of register y. In step SD13, it is determined whether or not the value of register y is greater than the maximum value of Y, and if the result is "NO", then the procedure returns to step SD2. Then, repeat the procedure in steps SD2-SD12 is repeated until register y becomes greater than the maximum of Y. When the value of register y becomes greater than the maximum of Y, the result in step SD13 becomes "YES", and the procedure ends. Of course, the image synthesis method mentioned above in Embodiment 2 may be written as a program. stored in a memory apparatus such as a ROM, and run by a CPU, but it is also possible to build it Into the hardware. Embodiment 3 Next, the third embodiment of the invention is explained. In Embodiment 3, synthesized image P is constructed by synthesizing N (N is a natural number greater than 1) video images V 1 through V N and M (M is a natural number) still images S 1 through S M , each of which are made up of (X×Y) (X and Y are natural numbers) pixels at arbitrary synthesis ratios. First, a synthesized still image is constructed by synthesizing M still images S 1 through S M each of which are made up of (X×Y) (X and Y are natural numbers) pixels at their respective synthesis ratios of w s1 :w s2 :...:w s(M-1) :w sM (with the conditions that w s1 +w s2 +...+w s(M-1) +w sM =1; w s1 , w s2 ,..., w s(M-1) , w sM ≧0). In this case, the pixels which are positioned on the coordinates (x,y) (1≦x≦X, 1≦y≦Y, x and y are natural numbers) on each still image S m (11≦n≦M, m is a natural number) and synthesized still image S are designated s mxy and s xy respectively, and the luminance of pixel s mxy is designated I smxy (01≦I smxy ≦1; wherein 0 black, 1 = white). In addition, for each still image S m , a threshold value T sm (01≦T sm ≦1) is set for the pixel luminance. Next, a synthesized video image is constructed by synthesizing N video images V 1 through V N each of which are made up of (X×Y) pixels at their respective synthesis ratios of w v1 : w v2 :...:w v(N-1) :w vN (with the conditions that w v1 +w v2 +...+w v(N-1) +w vN =1; w v1 ,w v2 ,...,w v(N-1) , w vN ≧0). in this case, the pixels which are positioned on the coordinates (x,y) on each video image V n (11≦n≦N, n is a natural number) are designated v nxy , and the luminance of pixel v nxy is designated I vnxy (01≦I vnxy ≦1; wherein 0 = black, 1 = white). In addition, for each video image V n , a threshold value T vn (01≦T vn ≦1) is set for the pixel luminance. Next, the image synthesis method used in the third embodiment of this invention is explained, using the flow charts in Figs. 5A through SE as references. First, in step SE1, it is determined whether or not M is equal to "1", that is, whether or not there is only one still image. If the result is "NO", then the procedure advances to step SE2. In step SE2, the initial value of the register y which varies the y coordinates is set at "1", and in step SE3, the initial value of register x which varies the x coordinates is set at "1". Next, in step SE4, the initial value of the variable m which indicates the order of the still images is set at "1", the initial value of the variable c which represents the number of pixels with a luminance greater than or equal to the threshold T sm is set at "0", the initial value of the sum w of the synthesis ratios of the pixels which have a luminance greater than or equal to the threshold T sm is set at "0", and then the procedure moves on to step SE5. In step SE5, it is determined whether or not the luminance I smxy of the pixel s mxy which is positioned at coordinates (x,y) on the relevant still image S m is greater than or equal to the threshold value T sm . If the result is "YES", then the procedure advances to step SE6. In step SE6, "1" is added to variable c. In step SE7, the synthesis ratio w sm of the relevant pixel s mxy is added to the total synthesis ratio w, then the procedure advances to step SE8. If the result of step SE5 is "NO", that is, if the luminance I smxy of pixel s mxy is less than the threshold value T sm then the procedure advances to step SE8 as well. In step SE8, "1" is added to variable m, and in step SE9, it is determined whether or not variable m is greater than the number M of images, and if the result is "NO", then the procedure returns to step SE5. Then, the procedure in steps SE5-SE8 is repeated for each pixel s mxy positioned at the coordinates (x,y) on every still image. That is, for the coordinates (x,y) on each still image S m , the number c of pixels s mxy which have a luminance I smxy greater than or equal to the threshold T sm and the sum w of their synthesis ratios are calculated. Then, for every pixel s mxy which is positioned on the coordinates (x,y) for every still image, the procedure given above in steps SE5-SE8 ends, the value of the variable m becomes greater than M, the result in step SE9 becomes "YES", and the procedure advances to step SE10. In step SE10, the initial value of variable m is set at "1", and the initial value of s which indicates value of the synthesis of the M pixels s mxy which are positioned at the coordinates (x,y) on the M still images, is set at "0". In step SE11, it is determined if variable c is equal to "0" or if it is equal to M; that is. if the luminances of the M pixels which are positioned at the coordinates (x,y) of the M still images is less than the threshold T sm (c = 0), or else if it is greater than or equal to the threshold T sm (c = M). If the answer is "YES", then the procedure advances to step SE12. In step SE12, the s is created by using equation (6), and then the procedure moves on to step SE13. s=s+w sm s mxy In step SE13, "1" is added to variable m, and in step SE14, it is determined whether or not the variable m is greater than the number M of still images, and if the result is "NO", then the procedure returns to step SE12. Then, the procedure in steps SE12 and SE13 is repeated for each pixel s mxy positioned at the coordinates (x,y) for every still image. That is, all pixels s mxy positioned at coordinates (x,y) on every still image are synthesized at their respective ratios w sm . Then, the procedure given above in steps SE12 and SE13 for each pixel s mxy positioned at the coordinates (x,y) for every still image ends, the value of variable m becomes greater than M, the result of step SE14 becomes "YES", and the procedure advances to step SE20. On the other hand, if the result of step SE11 is "NO", that is, if variable c is neither "0" nor M, then the procedure advances to step SE15. In step SE15, the average value W (=w/c) of the synthesis ratios with regard to the pixels having luminances greater than or equal to the threshold T sm is calculated, and in step SE16, it is determined whether or not the luminance I smxy of each pixel s mxy positioned at coordinates (x,y) on the relevant still image S m is less than the threshold value T sm . If the result is "YES", then the procedure advances to step SE17. In step SE17, the s is created by using equation (7) and then the procedure goes on to step SE18. s=s+(W+w sm )s mxy Of course, if the result of step SE16 is "NO", that is, in the case that the luminance I smxy of the pixel s mxy is greater than or equal to the threshold value T sm , then the procedure advances to step SE18 as well. In step SE18, "1" is added to variable m, and in step SE19, it is determined whether or not variable m is greater than the number M of still images. If the result is "NO", then the procedure returns to step SE16. Then, the procedure in steps SE16-SE18 is repeated for each pixel s mxy lying on the coordinates (x,y) of every still image. Then, the procedure given above in steps SE16-SE18 is completed for each pixel s mxy lying on the coordinates (x,y) of every still image, the value of variable m becomes greater than M, the result of step SE19 becomes "YES", and the procedure advances to step SE20. In step SE20, the synthesized pixel s xy which is positioned at the coordinates (x,y) on the synthesized image S is set equal to the s, then the procedure advances to step SE21. In step SE21, "1" is added to the value of register x. In step SE22, it is determined whether or not the value of register x is greater than the maximum value of X, and if the result is "NO", then the procedure returns to step SE4. Then, the procedure in steps SE4-SE21 is repeated until the value of x becomes greater than the maximum value X. When the value of the register x becomes greater than the maximum value X, the result of step SE22 becomes "YES", and the procedure advances to step SE23. In step SE23, "1" is added to the value of register y. In step SE24, it is determined whether or not the value of register y is greater than the maximum value Y, and if the result is "NO", then the procedure returns to step SE3. Then, the procedure in steps SE3-SE23 is repeated until the value of register y becomes greater than Y. When the value of register y becomes greater than the maximum value Y, the result of step SE24 becomes "YES", and the procedure advances to step SE26 in Fig. 5B. In step SE26, the initial value of register y is set at "1", and in step SE27, the initial value of register x is set at "1". Then, in step SE28, the initial value of the variable n which indicates the order of the video images is set at "1", the initial value of the variable c which represents the number of pixels with a luminance greater than or equal to the threshold T vn is set at "0", and the initial value of the sum w of the synthesis ratios of the pixels with a luminance above the threshold T vn is set at "0", then the procedure moves on to step SE29. In step SE29, it is determined whether or not the luminance I vnxy of pixel v nxy lying on the coordinates (x,y) on the relevant video image V n is greater than or equal to the threshold value T vn . If the result if "YES", then the procedure advances to step SE30. In step SE30, "1" is added to the variable c. In step SE31, the synthesis ratio w vn of the relevant pixel v nxy is added to the sum of the synthesis ratios and then the procedure advances to step SE32. If the result in step SE29 is "NO", that is, if the luminance I vnxy of the pixel v nxy is less than the threshold value T vn , then the procedure goes to step SE32 as well. In step SE32, "1" is added to the variable n, and in step SE33, it is determined whether or not variable n is greater than the number N of images; if the result is "NO", then the procedure returns to step SE29. Then, the procedure in steps SE29-SE32 is repeated for each pixel v nxy lying on the coordinates (x,y) on every video image. Thus, for the coordinates (x,y) on each video image V n , the number c of pixels v nxy with a luminance I vnxy greater than or equal to the threshold value T vn and the sum w of their respective synthesis ratios are calculated. Then, the procedure given above for steps SE29-SE32 is completed for each pixel v nxy located at the coordinates (x,y) on every video image, the value of variable n becomes greater than N, the result in step SE33 becomes "YES", and the procedure advances to step SE34. In step SE34, the initial value of variable n is set at "1". and the initial value of v which indicates value of the synthesis of N pixels v nxy which lie on the coordinates (x,y) for the N video images, is set at "0". In step SE35, it is determined whether variable c is equal to "0", equal to N, or equal to neither "0" nor N; that is, whether the luminance of all N pixels positioned at the coordinates (x,y) for the N video images is less than the threshold T vn (c = 0), greater than or equal to the threshold T vn (c = N), or is neither. Regarding step SE35, if variable c is shown to be equal to "0", then the procedure advances to step SE36. In step SE36, subroutine 1, as shown in Fig. 5C, is carried out. Regarding Fig. 5C. in step SE301, the v is created by utilizing equation (8), then the procedure advances to step SE302. v=v+w vn v nxy In step SE302, "1" is added to variable n, then in step SE303, it is determined whether or not variable n is greater than the number N of video images, and if the result is "NO", then the procedure returns to step SE301. Then, the procedure in steps SE301 and SE302 is repeated for each pixel v nxy positioned at the coordinates (x,y) for every video image. That is, the pixels v nxy which are positioned at the coordinates (x,y) on every video image are synthesized at their respective ratios w vn . Then, the procedure given above in steps SE301 and SE302 is completed for each pixel v nxy positioned at the coordinates (x,y) on every video image, the value of variable n becomes greater than N, the result of step SE303 becomes "YES", and the procedure advances to step SE39 in Fig. 5B. On the other hand, regarding step SE35 in Fig. 5B, if it is determined that the variable c is equal to N, then the procedure advances to step SE37. In step SE37, subroutine 2 as given in Fig. 5D is carried out. Regarding Fig. 5D, after the v is equal to the pixel s xy of synthesized still image S, the procedure advances to step SE39 in Fig. 5B. Regarding step SE35 in Fig. 5B, if the variable c is determined to be neither "0" nor N, then the procedure advances to step SE38. In step SE38, subroutine 3 given in Fig. 5E is carried out. Regarding Fig. 5E, in step SE321, the average value W (=w/c) of the synthesis ratios with regard to the pixels having luminance greater than or equal to T nv is calculated, then in step SE322, it is determined whether or not the luminance I vnxy of the pixel v nxy positioned at coordinates (x,y) of the relevant video image V n is less than the threshold T vn . If the answer is "YES", then the procedure advances to step SE323. In step SE323, the v is created by using equation (9), and then the procedure advances to step SE324. v = v+(W+w vn )v nxy Of course, if the result in step SE322 is "NO", that is, if the luminance I vnxy of pixel v nxy is greater than or equal to the threshold T vn , then the procedure advances to step SE324. In step SE324, "1" is added to the variable n, and then in step SE325, it is determined whether or not the variable n is greater than the number N of video images. If the result is "NO", then the procedure returns to step SE 322. Then, the procedure in steps SE322-SE324 is repeated for each pixel v nxy lying on the coordinates (x,y) of every video image. Then, the procedure given above in steps SE322-SE324 is completed for each pixel v nxy lying on the coordinates (x,y) of every video image, the value of the variable N becomes greater than N, the result in step SE325 becomes "YES", and the procedure advances to step SE39. In step SE39, the synthesized pixel v xy which lies on the coordinates (x,y) of the synthesized image V is set equal to v, then the procedure advances to step SE40. In step SE40, "1" is added to the register value x. In step SE41, it is determined whether or not the register value x is greater than the maximum of X, and if the answer is "NO", then the procedure returns to step SE28. Then, the procedure in steps SE28-SE40 is repeated until the register value x becomes greater than the maximum value X. When the register value x becomes greater than the maximum value X, the result in step SE41 becomes "YES", and the procedure advances to step SE42. In step SE42, "1" is added to the register value y. In step SE43, it is determined whether or not the register value y is greater than the maximum Y, and if the answer is "NO", then the procedure returns to step SE27. Then the procedure in steps SE27-SE42 is repeated until the register value y becomes greater than the maximum Y. When the register value y becomes greater than the maximum value Y, the result in step SE43 becomes "YES", and the procedure ends. In the aforementioned Embodiment 3, an example was given (the procedure in Fig. 5A step SE17) in which, when synthesizing the pixels of the still images, the synthesis ratios of the non-synthesizing pixels was averaged and distributed. However, regarding the procedure in step SE17, by using equation (10) it is possible to distribute the synthesis ratios of the non-synthesizing pixels proportionally with the synthesis ratios of the synthesizing pixels. Next, in the aforementioned Embodiment 3, the procedure is explained in the case in which N=2 and M=1, that is, if the creation of a synthesized image V which is made by combining two video images V 1 and V 2 and one still image S is desired, by following the flow chart in Fig. 6. First, in step SF1, the initial value of the register y to vary the y coordinates is set at "1", and likewise, in step SF2, the initial value of the register x to vary the x coordinates is set at "1". Next, in step SF3, it is determined whether or not the luminance I v1xy of pixel v 1xy lying on the coordinates (x,y) of the video image V 1 is greater than or equal to the threshold value T 1 , and the luminance I v2xy of pixel v 2xy is less than the threshold value T 2 . If the result is "YES", then the procedure advances to step SF4. In step SF4, the synthesized pixel v xy which lies on the coordinates (x,y) of the synthesized image V xy is set equal to pixel v 2xy of video image V2, then the procedure moves on to step SF5. In step SF5, it is determined whether or not the luminance I v1xy of pixel v 1xy is less than the threshold value T 1 and the luminance I v2xy of pixel v 2xy is greater than or equal to the threshold T 2 . If the result is "YES", then the procedure advances to step SF6. In step SF6, synthesized pixel v xy is set equal to pixel v 1xy , then the procedure goes to step SF10. On the other hand, if the result in step SF5 is "NO", that is, if the luminance I v1xy of pixel v 1xy is greater than or equal to the threshold T 1 or the luminance I v2xy of pixel v 2xy is less than the threshold T 2 , then the procedure moves on to step SF7. In step SF7, it is determined whether or not the luminance I v1xy of pixel v 1xy is greater than or equal to the threshold value T 1 and the luminance I v2xy of pixel v 2xy is greater than or equal to the threshold T 2 . If the result is "YES", then.the procedure advances to step SF8. In step SF8, synthesized pixel v xy is set equal to pixel s xy and then the procedure advances to step SF10. On the other hand, if the result in step SF7 is "NO", that is, if the luminance I v1xy of pixel v 1xy is less than the threshold T 1 or the luminance I v2xy of pixel v 2xy is less than the threshold T 2 , then the procedure advances to step SF9. In step SF9, the synthesized pixel v xy is created using equation (11), and then the procedure advances to step SF10. v xy =w 1 v 1xy +w 2 v 2xy In step SF10, "1" is added to the value of register x. In step SF11, it is determined whether or not the value of register x is greater than the maximum value X, and if the result is "NO". then the procedure returns to step SF3. Then, the procedure in steps SF3-SF10 is repeated until the value of register x becomes greater than the maximum value X. When the value of register x becomes greater than the maximum X, then the result in step SF11 becomes "YES", and the procedure advances to step SF12. In step SF 12, "1" is added to the register value y. In step SF13, it is determined whether or not the value of register y is greater than the maximum value Y, and if the result is "NO", then the procedure returns to step SF2. Then the procedure in steps SF2-SF12 is repeated until the value of register y becomes greater than the maximum value Y. When the value of register y becomes greater than the maximum Y, then the result in step SF13 becomes "YES", and the procedure ends. Next, in the aforementioned Embodiment 3, the procedure in the case that M=1, that is, if a synthesized image V is to be created by synthesizing N video images V 1 -V N and a single still image S, is explained. The N video images V 1 -V N are combined at their respective synthesis ratios w 1 :w 2 :...:w (N-1) :w N (with the conditions w 1 +w 2 +...+w (N-1) +w N =1; w 1 , w 2 ,...,w (N-1) , w N ≧0). In this case, the pixel corresponding to the coordinates (x,y) on each video image V n (1≦n≦N, n is a natural number) is designated v nxy , and the luminance of pixel v nxy is designated I vnxy (0≦I vnxy ≦1; wherein 0 = black, 1 = white). Also, the threshold value T n (0≦T n ≦1) is set for the luminances of the pixels of each video image V n . Figs. 7A-7D represent a flow chart which shows the procedure for the case in which a synthesized image is constructed by synthesizing N video images V 1 -V N and a single still image S. In Figs. 7A-7D, those procedures which correspond to the ones which were in Figs. 5A-5E are given the same symbols. The procedures in the flow chart in Figs. 7A-7D, when not synthesizing M still images S 1 -S M , are identical to those in the flow chart in Figs. 5A-5E, so the explanation is omitted. The image synthesis method described above in Embodiment 3 is able to be written as a program, stored in a memory apparatus such as a ROM, and run by a CPU; it is also possible to build it into the hardware.40 Embodiment 4 In the following. the Embodiment 4 of the present invention will be explained. In the Embodiment 4, synthesized image P is constructed by synthesizing N (N is a natural number greater than 1) video images V 1 -V N and M (M is a natural number) still images S 1 -S M , each of which are made up of (X×Y) (X and Y are natural numbers) pixel. First, a synthesized still image S is constructed by selecting a pixel with the lowest luminance among those pixels that lie on the same coordinates of the still images S 1 -S M . Next, a synthesized image P is constructed by synthesizing video images V 1 -V N and the synthesized still image S. In the case where all the luminances of pixels that lie on the same coordinates of the video images V 1 -V N are greater than predetermined threshold, the pixel that lie on the same coordinate of the still image S is selected as a synthesized still pixel of the synthesized image P. In other case, the pixel with the lowest luminance among those pixels that lie on the same coordinates of the video images V 1 -V N is selected as a synthesized pixel. In this case, the pixels that lie on the coordinates (x,y) (1≦x≦X, 1≦y≦Y; wherein x and y are natural numbers) of each still image S m (1≦n≦M; m is a'natural number) and synthesized still image S are designated as pixel s mxy and s xy , respectively. The luminance of pixel s mxy is represented by I smxy (0≦I smxy ≦1; wherein 0 = black and 1 = white). In addition, the pixel which lie on the coordinates (x,y) of each video image V n (1≦n≦N; n is a natural number) is designated as V nxy , and the luminance of this pixel is represented by I vnxy (0≦I vnxy ≦1; wherein 0 = black and 1 = white). Furthermore, a luminance threshold T vn (01≦T vn ≦1) is set for each video image V n In the following, the image synthesizing process according to the Embodiment 4 of the present invention will be explained with reference to the flow chart in Figs. 8A and 8B. Initially, in step SG1, a decision is made as to whether or not M is "1", in other words, whether or not only one still image exists. In the case when the result of this decision is "NO", the process moves to step SG2. In step SG2, the initial value of register y for changing the y coordinates is set to "1". In step SG3, the initial value of register x for changing the x coordinates is similarly set to "1". Subsequently, in step SG4, the initial value of variable m indicating the order of the still images is set at "2", the initial value of the lowest luminance i is set equal to luminance I s1xy of pixel s 1xy , and the initial value of s indicating value of synthesized pixel is set equal to pixel s 1xy , then the procedure moves on to step SG5. In step SG5, a decision is made as to whether luminance I smxy of pixel s mxy is less than luminance i or not. In the case when the result of this decision is "YES", the process moves to step SG6. In step SG6, luminance i is set equal to luminance I smxy , and the process then moves to step SG7. In step SG7, after the s is set equal to the pixel s mxy , the process moves to step SG8. In step SG8, "1" is added to variable m: in step SG9, a decision is made as to whether or not variable m is greater than the number M of still images. In the case when the result of this decision is "NO", the process returns to step SG5. The procedure described in steps SG5-SG8 is then repeated with regard to pixels s mxy located at the coordinates (x,y) of all still images. After the procedure of steps SG5-SG8 has been completed on pixels s mxy located at the coordinates (x,y) of all still images, and the value of variable m is greater than M, the decision result of step SG9 becomes "YES", thereby allowing the process to proceed to step SG10. In step SG10, pixel s xy located at coordinates (x,y) of synthesized image S is set equal to s, and the process then moves to step SG11. In step SG11, "1" is added to the value of register x. In step SG12, a decision is made as to whether or not the value of register x is greater than maximum value X. In the case when the result of this decision is "NO", the process returns to step SG4. The procedure of steps SG4-SG11 is then repeated until the value of register x becomes greater than maximum value X. When the value of register x becomes greater than maximum value X, the result of the decision in step SG12 becomes "YES", and the process moves to step SG13. In step SG13, "1" is added to the value of register y. In step SG14, a decision is made as to whether or not the value of register y is greater than maximum value Y. In the case when the result of this decision is "NO", the process returns to step SG3. The procedure of step SG3-SG13 is then repeated until the value of register y becomes greater than maximum value Y. When the value of register y becomes greater than maximum value Y, the result of the decision in step SG14 becomes "YES", and the process moves to step SG16. In addition, when the decision result of step SG1 is "YES", in other words, M is "1", only one still image exists, hence the process skips to step SG15. In step SG15, after the still image S 1 is designated as synthesized still image S, the process moves to step SG16. In steps SG16 and SG17, the initial value of "1" is given to "x" and "y". In step SG18, a decision is made as to whether or not all the luminance I v1xy -I vNxy of pixels v 1xy -v Nxy of each video image V 1 -V N are greater than corresponding threshold values T v1 -T vN . In the case when this decision result is "YES", the process moves to step SG25. In step SG25, the process ends after synthesized pixel v xy is set equal to the pixel s xy of synthesized still image S. On the other hand, when the decision result of step SG18 is "NO", the process moves to step SG19. In step SG19, the initial value of variable n is set equal to "1", and the initial value of the lowest luminance i is similarly set equal to "1". Following this, the process moves to step SG20. In step SG20, a decision is made as to whether or not luminance I vnxy is less than or equal to threshold T vn , and whether or not luminance I vnxy is less than luminance i. In the case when the result of this decision is "YES", the process moves to step SG21. In step SG21. luminance i is set equal to luminance I vmxy , and the process moves to step SG22. In step SG22, a pixel v xy is set equal to synthesized pixel v nxy , and the process then moves to step SG23. In step SG23, "1" is added to variable n. Subsequently, in step SG24, a decision is made as to whether or not variable n is greater than the number N of video images. In the case when the result of this decision is "NO", the process returns to step SG20. The procedure of steps SG20-SG23 is then repeated for each pixel v nxy located at coordinates (x,y) of each video image. When the procedure of step SG20-SG23 has been completed with regard to each pixel v nxy located at coordinates (x,y) of each video image, the value of variable n becomes greater than N, the decision result of step SG24 becomes "YES", and the process ends. In step SG26, "1" is added to the value in register "x". In step 27, a judgment is made as to whether the value "x" is greater than the maximum value "X" or not. If the Judgment is "NO", the procedure returns to step SG18. Then, the procedures SG18-SG26 are repeated until the value "x" becomes greater than the value "X". If the value "x" becomes greater than the value "X" and the judgment is "YES", the procedure proceeds to step SG28. In step SG28, "1" is added to the value of register "y". In step SG29, a judgment is made as to whether the value "y" is greater than the maximum value "Y" or not. If the judgment is "NO", the procedure returns to step SG17. Then, the procedures SG17-SG28 are repeated until the value "y" becomes greater than the value "Y". The procedure is terminated when the value "y" becomes greater than the value "Y" and the judgment is "YES". Furthermore, the image synthesizing process according to the aforementioned Embodiment 4 can be achieved by means of pre-recording a program in a memory apparatus such as a ROM and the like, and then running this program using a CPU. In addition. the same process can be performed by means of hardware as well. Embodiment 5 In the following, the Embodiment 5 of the present invention will be explained. In the Embodiment 5 of the present invention, synthesized image P is constructed by synthesizing N (N is a natural number greater than 1) video images V 1 -V N and M (M is a natural number) still images S 1 -S M , each of which are made up of (X×Y) (X and Y are natural numbers) pixel. First. a synthesized still image S is constructed by selecting a pixel with the lowest luminance among those pixels which lie on the same coordinates of the still images S 1 -S M . Next, a pixel possessing the lowest luminance among those pixels which lie on the same coordinates of the video images V 1 -V N is selected. In the case where the luminance is greater than a predetermined threshold, the pixel which lie on the same coordinates of the still image S is selected as a pixel corresponding same coordinate of the synthesized image. In other case, the aforementioned pixel possessing the lowest luminance is selected as a pixel corresponding same coordinate of the synthesized image. In this case, the pixels which lie on the coordinates (x,y) (1≦x≦X, 1≦y≦Y; wherein x and y are natural numbers) of each still image S m (1≦n≦M; m is a natural number) and synthesized still image S are designated as pixel s mxy and s xy , respectively. The luminance of pixel s mxy is represented by I smxy (0≦I smxy ≦1; wherein 0 = black and 1 = white). In addition, the pixel which lies on the coordinates (x,y) of each video image V n (1≦n≦N: n is a natural number) is designated as V nxy , and the luminance of this pixel is represented by I vnxy (0≦I vnxy ≦1; wherein 0 = black and 1 = white). Furthermore, a luminance threshold T v (01≦T v ≦1) is set for video image. In the following, the image synthesizing process according to the Embodiment 5 of the present invention will be explained with reference to the flow chart in Fig. 9. In this figure, procedures corresponding to those described in Fig. 8 will be denoted by the same character (numeral) and their explanations will be omitted. In other words, in the case when only one still image S 1 exists, this image is designated as synthesized still image S; in the case when a plurality of still images exist, the pixel with the lowest luminance among pixels of all still images S 1 -S M located at the same coordinates is designated as synthesized pixel s xy corresponding to same coordinate of synthesized still image S. This process is identical to that described in the aforementioned Embodiment 4. In step SG16, the initial value of register y for changing the y coordinate is similarly set to "1". In step SG17, the initial value of register x for changing the x coordinates is similarly set to "1". Subsequently, in step SG30, the initial value of variable n indicating the order of the video images is set equal to "2", the initial value of the minimum luminance i is set equal to luminance I v1xy of pixel v 1xy , and the initial value of v that represents the pixel possessing the lowest luminance corresponding to coordinate (x,y) is set equal to pixel v 1xy . Following these actions, the process moves to step SG31. In step SG31, a decision is made as to whether luminance I vnxy of pixel v nxy is less than luminance i or not. In the case when the result of this decision is "YES", the process moves to step SG32. In step SG32, luminance l is set equal to luminance I vnxy , and the process then moves to step SG33. In step SG33, after v is set equal to pixel v nxy , the process moves to step SG34. In step SG34, "1" is added to variable n; in step SG35, a decision is made as to whether or not variable n is greater than the number N of video images. In the case when the result of this decision is "NO", the process returns to step SG31. The procedure described in steps SG31-SG34 is then repeated with regard to pixels v nxy located at the coordinates (x,y) of all video images. After the procedure of steps SG31-SG34 has been completed on pixels v nxy located at the coordinates (x,y) of all video images, and the value of variable n is greater than N, the decision result of step SG35 becomes "YES", thereby allowing the process to proceed to step SG36. In step SG36, a decision is made as to whether or not luminance i is greater than predetermined threshold T v . In the case when the result of this decision is "NO", the process moves to step SG37. In step SG37, pixel v xy located at coordinates (x,y) of synthesized image V is set equal to v, and the process then skips to step SG26. On the other hand, in the case when the decision result of step SG36 is "YES", in other words, when luminance 1 is greater than threshold T v , the process moves to step SG38. In step SG38, synthesized pixel v xy is set equal to pixel s xy of synthesized still image S, and the process then moves to step SG26. In step SG26, "1" is added to the value of register x. In step SG27, a decision is made as to whether or not the value of register x is greater than maximum value X. In the case when the result of this decision is "NO", the process returns to step SG30. The procedure of steps SG30-SG26 is then repeated until the value of register x becomes greater than maximum value X. When the value of register x becomes greater than maximum value X, the result of the decision in step SG27 becomes "YES", and the process moves to step SG28. In step SG28, "1" is added to the value of register y. In step SG29, a decision is made as to whether or not the value of register y is greater than maximum value Y. In the case when the result of this decision is "NO", the process returns to step SG29. The procedure of step SG17-SG28 is then repeated until the value of register y becomes greater than maximum value Y. When the value of register y becomes greater than maximum value Y, the result of the decision in step SG29 becomes "YES", which in turn allows the process to end. Furthermore, the image synthesizing process according to the aforementioned Embodiment 5 can be achieved by means of prerecording a program in a memory apparatus such as a ROM and the like, and then running this program using a CPU. In addition, the same process can be performed by means of hardware as well. Embodiment 6 In the following, the Embodiment 6 of the present invention will be explained. Fig. 10 is a block diagram showing the structure of an image synthesizing apparatus according to the Embodiment 6. In Fig. 10, the image synthesizing apparatus is formed from an N number (N is a natural number greater than 1) of input terminals 21 1 -21 N , an N number of luminance signal separators 22 1 -22 N , an N number of input terminals 23 1 -23 N , comparative computing unit 24, image switch 25, ratio adjuster 26, adder 27, and output terminal 28. An N number of images P 1 -P N are inputted from input' terminals 21 1 -21 N , respectively, and supplied to the corresponding luminance signal separators 22 1 -22 N . Each luminance signal separator 22 1 -22 N then supplies an inputted image P 1 -P N , respectively, to image switch 25. In addition, the aforementioned luminance signal separators 22 1 -22 N separate luminance signals I P1 -I PN from images P 1 -P N and supply them to comparative computing unit 24. An N number of threshold signals S T1 -S TN are inputted from input terminals 23 1 -23 N , respectively, and supplied to comparative computing unit 24. Threshold signals S T1 -S TN correspond to thresholds T 1 -T N of luminances of pixels previously set at each image P 1 -P N . Comparative computing unit 24 compares each luminance signal I P1 -I PN with the corresponding threshold signal ST 1 -S TN , and then supplies natural numbers k corresponding to all luminance signals I Pk which are less than or equal to the corresponding threshold signal S Tk (1≦k≦N) to image switch 25. In the case when all luminance signal I P1 -I PN of N video images P 1 -P N are greater than the corresponding threshold signals S T1 -S TN , this information is reported to image switch 25. In the case when such information is reported, i.e., information that all luminance signals I P1 -I PN are greater than threshold signal S T1 -S TN , images switch 25 supplies all video images P 1 -P N to ratio adjuster 26; in all other cases, image switch 25 supplies all video images P k corresponding to supplied natural numbers k to ratio adjuster 26. Ratio adjuster 26 redistributes the predetermined synthesis ratios of the not inputted visual images over those of the inputted visual images. Inputted video images P k are multiplied by a sum of the predetermined synthesis ratio and the redistributed syntheses ratio. And the resultant video images P k ' are outputted to adder 27. Video images P k ' are added by adder 27 and the sum is outputted through terminal 28 as a synthesized image P. Embodiment 7 In the following, the Embodiment 7 of the present invention will be explained. Fig. 11 is a block diagram showing a structure of an image synthesizing apparatus according to the Embodiment 7. In Fig. 11, the image synthesizing apparatus is formed from input terminals 29 1 , 29 2 , 30 1 , and 30 2 ; partial resistor 31; operational amplifier 32; comparator 33 1 and 33 2 ; Schmitt trigger 34 1 and 34 2 ; delay lines 35 1 -35 3 ; analog switch 36; and output terminal 37. Image signals S P1 , S P2 of images P 1 , P 2 are respectively inputted from input terminals 29 1 , 29 2 , and supplied to one input terminal of corresponding comparators 33 1 , 33 2 . On the other hand, the two threshold signals S T1 , S T2 are inputted from input terminals 30 1 , 30 2 , respectively, and then supplied to the other terminals of comparators 33 1 , 33 2 , respectively. Threshold signals S T1 , S T2 correspond to luminance thresholds T 1 , T 2 previously designated for images P 1 , P 2 . Comparators 33 1 , 33 2 compare, respectively, the luminance signals of each image signal S P1 , S P2 with the corresponding threshold signals S T1 , S T2 . In the case when each luminance signal is greater than its respective corresponding threshold signal S T1 . S T2 , a comparative signal of level "H" is outputted, while in the case when these aforementioned signals are less than or equal to their threshold signals, a comparative signal of level "L" is outputted. Each comparative signal is subsequently shaped into a waveform by means of Schmitt triggers 34 1 , 34 2 and then supplied to control terminals C0, C1 of analog switch 36. Image signals S P1 , S P2 are supplied to input terminals I 1 , I 2 of analog switch 36 via delay lines 35 1 , 35 2 each possessing a predetermined delay, and one-half of each signal is respectively summed by means of partial resister 31. These signals then undergo "buffering" by means of operational amplifier 32 of amplification ratio 1, and are then supplied to input terminal I3 of analog switch 36 via delay line 35 3 also possessing a predetermined delay. Analog switch 36 then respectively sets the values of control data DC0, DC1 to "1", respectively, in the case when each of the comparative signals inputted from control terminal C0, C1 is at level "H", or sets the values of control data DC0, DC1 to "0", respectively, in the case when each signal displays a level "L". In the case when the values of control data DC0, DC1 read "00", the output signal of delay line 35 3 inputted from input terminal I0 is selected; in the case when the aforementioned values read "01", the output signal of delay line 35 1 inputted from input terminal I1 is selected; in the case when the aforementioned values read "10", the output signal of delay line 35 2 inputted from input terminal I2 is selected; and in the case when the aforementioned values read "11", the output signal of delay line 35 3 input from input terminal I3 is selected. The selected output signal is then outputted from output terminal 37 as synthesized image signal S P . Furthermore, delay lines 35 1 -35 3 are provided for the purpose of adjusting the input time difference between the analog signal and the aforementioned comparative signal inputted to analog switch 36. However, in the case when a high quality synthesized image signal S p is not in particular necessary, these structures may be omitted. Fig. 12 shows a structural example of an image communication system in which the image synthesizing apparatus according to the aforementioned Embodiment 7 is utilized. In Fig. 12, two image communication terminal units 41a, 41b are connected via communication circuit 42 in a manner such that interactive communication is possible (bidirectional communication). Image communication terminal units 41a, 41b comprise, respectively, image input units 43a, 43b; image synthesizing units 44a, 44b: and display units 45a, 45b. Image input units 43a, 43b respectively comprise a video camera or the like which captures the desktop of each user, and outputs images 46a, 46b, for example, as shown in Fig. 13. Image synthesizing apparatus 44a conducts synthesis such that image 46a, supplied directly from image input apparatus 43a, and image 46b, supplied from image input apparatus 43b via communication circuit 42, appear transparent and discernible. Image synthesizing apparatus 44a then supplies image 47 shown in Fig. 13 to display apparatus 45a. Image synthesizing apparatus 44b conducts similar synthesis such that image 46b, supplied directly from image input apparatus 43b, and image 46a, supplied from image input apparatus 43a via communication circuit 42, appear transparent and discernible, and then supplies image 47, shown in Fig. 13, to display apparatus 45b. According to the present embodiment, reduction of the contrast can be prevented, and synthesized images can be viewed clearly. Embodiment 8 (considered as being not part of invention) In the following, the Embodiment 8 of the present invention will be explained. Fig. 14 is a block diagram showing the structure of an image synthesizing apparatus according to the Embodiment 8. In Fig. 14, the aforementioned image synthesizing apparatus comprises an N number (N is a natural number greater than 1) of input terminals 51 1 -51 N , an N number of luminance signal separators 52 1 -52 N , comparative computing unit 53, image switch 54, and output terminal 55. An N number of Images P 1 -P N are inputted, respectively, from input terminals 51 1 -51 N , and are supplied to the corresponding luminance signal separator 52 1 -52 N . Each luminance signal separator 52 1 -52 N supplies an inputted image P 1 -P N to image switch 54, and at the same time, separates luminance signal I P1 -I PN from the image P 1 -P N , and supplies this luminance signal to comparative computing unit 53. Comparative computing unit 53 compares the levels of all luminance signals I P1 -I PN and reports the luminance signal which is the lowest level to image switch 54. Image switch 54 then outputs the image possessing the reported luminance signal as output image P from output terminal 55. In the Embodiment 8, in the case when N = 2, in other words, the case when two images P 1 and P 2 are synthesized by means of the image synthesizing apparatus is shown in Fig. 15. In Fig. 15, image synthesizing apparatus comprises input terminals 56 1 , 56 2 ; comparator 57; Schmitt trigger 58; delay lines 59 1 , 59 2 ; analog switch 60; and output terminal 61. Image signals S P1 , S P2 of the two images P 1 , P 2 are inputted respectively from input terminals 56 1 , 56 2 , and then supplied to the respective input terminal of comparator 57. Comparator 57 compares the luminance signals of image signals S P1 , S P2 , and outputs a comparative signal of level "H" in the case when luminance signal of image signal S P1 is greater than the luminance signal of image signal S P2 , or a comparative signal of level "L" in the case when the other situation occurs. Comparative signal is then shaped into a waveform by means of Schmitt trigger 58 and sent to control terminal C of analog switch 60. Image signals S P1 , S P2 are respectively supplied to input terminals I0, I1 of analog switch 60 via delay lines 59 1 , 59 2 each possessing a predetermined delay. In the case when an "L" level comparative signal is inputted from control terminal C, analog switch 60 selects the output signal inputted from input terminal I0; in the case when an "H" level comparative signal is inputted, analog switch 60 selects the output signal inputted from input terminal I1. The selected output signal is then outputted from output terminal 61 as output image signal SP. Furthermore, delay lines 59 1 , 59 2 are provided for the purpose of adjusting the input time difference between the analog signal and the aforementioned comparative signal inputted from analog switch 60. However, in the case when a high quality output image signal S p is not in particular necessary, these structures may be omitted. Fig. 16 shows a structural example of an image communication system in which the image synthesizing apparatus shown in Fig. 15 is utilized. In Fig. 16, two image communication terminal units 62a, 62b are connected via communication circuit 63 in a manner such that interactive communication is possible (bidirectional communication). Image communication terminal units 62a, 62b comprise, respectively, image input units 64a, 64b; image synthesizing units 65a, 65b; and display units 66a, 66b. Image input units 64a, 64b respectively comprise a video camera or the like which captures the desktop of each user, and outputs images 67a, 67b, for example, as shown in Fig. 17. Image synthesizing apparatus 65a conducts synthesis such that image 67a, supplied directly from image input apparatus 64a, and image 67b, supplied from image input apparatus 64b via communication circuit 63, appear transparent and discernible. Image synthesizing apparatus 65a then supplies image 68 shown in Fig. 17 to display apparatus 66a. Image synthesizing apparatus 65b conducts a similar synthesis such that image 67b, supplied directly from image input apparatus 64b, and image 67a, supplied from image input apparatus 64a via communication circuit 63, appear transparent and discernible, and then supplies image 68, shown in Fig. 17, to display apparatus 66b. According to the present embodiment, reduction of the contrast can be prevented, and synthesized images can be viewed clearly. Embodiment 9 In the following. the Embodiment 9 of the present invention will be explained. Fig. 18 is a block diagram showing the structure of an image synthesizing apparatus according to the Embodiment 9. In Fig. 18, the aforementioned image synthesizing apparatus comprises an M number (M is a natural number) of input terminals 71 1 -71 M and 72 1 -72 M ; an N number of input terminals 73 1 -73 N , and 74 1 -74 N ; an M number of luminance signal separators 75 1 -75 M ; an N number of luminance signal separators 76 1 -76 N : comparative computing units 77 and 78: image switches 79 and 80; ratio adjusters 81 and 82; adders 83 and 84; and output terminal 85. An M number of still images S 1 -S M are inputted, respectively, from input terminals 71 1 -71 M , and are supplied to the corresponding luminance signal separators 75 1 -75 M . Each luminance signal separator 75 1 -75 M supplies an inputted still image S 1 -S M to image switch 79, and at the same time, separates luminance signal I S1 -I SM from the respective still image S 1 -S M , and supplies this luminance signal to comparative computing unit 77. An M number of threshold signals S TS1 -S TSM are respectively inputted from input terminals 72 1 -72 M , and supplied to comparative computing unit 77. Threshold signals S TS1 -S TSM correspond to luminance thresholds T S1 -T SM of pixels previously set at each still image S 1 -S M . Comparative computing unit 77 compares each luminance signal I S1 -I SM with the corresponding threshold signal S TS1 -S TSM , and then supplies natural numbers k corresponding to all luminance signals I Sk which are less than or equal to the corresponding threshold signal S TSk (1≦≦M) to image switch 79. In the case when all luminance signals I S1 -I SM of M still images S 1 -S M are greater than the corresponding threshold signals S TS1 -S TSM , this information is reported to image switch 79. In the case when such information is reported, i.e., information that all luminance signals I S1 -I SM are greater than threshold signals S TS1 -S TSM , images switch 79 supplies all still images S 1 -S M to ratio adjuster 81; in all other cases, image switch 79 supplies all still images S k corresponding to supplied natural numbers k to ratio adjuster 81. Ratio adjuster 81 redistributes the predetermined synthesis ratios of not inputted visual image over those of the inputted visual images. Inputted visual images S k are multiplied by a sum of the predetermined synthesis ratio and the redistributed synthesis ratio. The products S' k are supplied to adder 83. Adder 83 adds still image S' k and the sum is supplied as a synthesized still image to visual switch 80. An N number of video images V 1 -V N are inputted, respectively, from input terminals 73 1 -73 N , and are supplied to the corresponding luminance signal separators 76 1 -76 N . Each luminance signal separator 76 1 -76 N supplies an inputted video image V 1 -V N to image switch 88, and at the same time. separates luminance signal I V1 -I VN from the respective video image V 1 -V N , and supplies this luminance signal to comparative computing unit 78. An N number of threshold signals S TV1 -S TVN are respectively inputted from input terminals 74 1 -74 N , and supplied to comparative computing unit 78. Threshold signals S TV1 -S TVN correspond to luminance thresholds T V1 -T VN of pixels previously set at each video image V 1 -V N . Comparative computing unit 78 compares each luminance signal I V1 -I VN with the corresponding threshold signal S TV1 -S TVN , and then supplies natural numbers k corresponding to all luminance signals I Vk which are less than or equal to the corresponding threshold signal S TVk (1≦k≦N) to image switch 80. In the case when all the luminance signals I V1 -I VN of N video images V 1 -V N are greater than the corresponding threshold signals S TV1 -S TVN , this information is reported to image switch 80. In the case when such information is reported, image switch 80 supplies the inputted synthesized still image S to adder 84; in all other cases, image switch 80 supplies all video images V k corresponding to supplied natural numbers k to ratio adjuster 82. Ratio adjuster 82 redistributes the predetermined synthesis ratios of the not inputted visual images over those of the inputted visual images. Inputted video image V k are multiplied by the sum of the above operation, and the resultant video image V' k are outputted to adder 84. Adder 84 outputs the synthesized still image S when a synthesized still image is inputted. Otherwise, video images V' k are added by adder 84 and the sum is outputted through terminal 85 as a synthesized image P. Embodiment 10 In the following. the Embodiment 10 of the present invention will be explained. Fig. 19 is a block diagram showing a structure of an image synthesizing apparatus according to the Embodiment 10. In Fig. 19, the image synthesizing apparatus is formed from input terminals 91, 92 1 , 92 2 , 93 1 , and 93 2 ; partial resistor 94; operational amplifier 95; comparator 96 1 and 96 2 : Schmitt trigger 97 1 and 97 2 ; delay lines 98 1 -98 4 ; analog switch 99; and output terminal 100. One image signal S s of still image S is inputted from input terminal 91 and supplied to input terminal I0 of analog switch 99 via delay line 98 1 possessing a predetermined delay. In addition, two image signals S V1 , S V2 of video images V 1 , V 2 are respectively inputted from input terminals 92 1 , 92 2 , and supplied to one input terminal of corresponding comparators 961, 962, respectively. On the other hand, the two threshold signals S T1 , S T2 are inputted from input terminals 93 1 , 93 2 , respectively, and then supplied to the other terminals of comparators 96 1 , 96 2 , respectively. Threshold signals S T1 , S T2 correspond to luminance thresholds T 1 , T 2 previously designated for video images V 1 , V 2 . Comparators 96 1 , 96 2 compare, respectively, the luminance signals of each image signal S V1 , S V2 with the corresponding threshold signals S T1 , S T2 . In the case when each luminance signal is greater than its respective corresponding threshold signal S T1 , S T2 , a comparative signal of level "H" is outputted, while in the case when these aforementioned signals are less than or equal to their threshold signals, a comparative signal of level "L" is outputted. Each comparative signal is subsequently shaped into a waveform by means of Schmitt triggers 97 1 , 97 2 and then supplied to control terminals C0, C1 of analog switch 99. Image signals S V1 , S V2 are supplied to input terminals I1, I2 of analog switch 99 via delay lines 98 2 , 98 3 each possessing a predetermined delay, and one-half of each signal is respectively summed by means of partial resister 94. The resultant signal then undergoes "buffering" by means of operational amplifier 95 of amplification ratio 1, and is then supplied to input terminal I3 of analog switch 99 via delay line 98 4 also possessing a predetermined delay. Analog switch 99 then respectively sets the values of control data DC0, DC1 to "1", in the case when each of the comparative signals inputted from control terminal C0, C1 is at level "H", or sets the values of control data DC0, DC1 to "0", respectively, in the case when each signal displays a level "L". In the case when the values of control data DC0, DC1 read "11", the output signal of delay line 98 1 inputted from input terminal I0 is selected; in the case when the aforementioned values read "01", the output signal of delay line 98 2 inputted from input terminal I1 is selected: in the case when the aforementioned values read "10", the output signal of delay line 98 3 inputted from input terminal I2 is selected; and in the case when the aforementioned values read "00", the output signal of delay line 98 4 input from input terminal I3 is selected. The selected output signal is then outputted from output terminal 100 as synthesized image signal S V . Furthermore, delay lines 98 1 -98 4 are provided for the purpose of adjusting the input time difference between the analog signal inputted from analog switch 99 and the aforementioned comparative signal. However, in the case when a high quality synthesized image signal S V is not in particular necessary, these structures may be omitted. In addition, in the case when two still images are provided, i.e., as in the number of video images provided, a circuit similar to the circuit relating to image signals S V1 , S V2 of video images V 1 , V 2 in Fig. 19 is provided, and the luminance signals of each still image are respectively compared with previously set threshold signals by means of a comparator. Synthesis is then conducted at a predetermined synthesis ratio using a partial resistor or the like, and image signals S V1 , S V2 of video images V 1 , V 2 are inputted along with the image signals of the still images into an analog switch which has been expanded to contain two more input terminals and two more control terminals. Based on the comparative signal inputted from the control terminal, in the case when an image signal of a still image is selected, and when the luminance signals of the two still images are either greater or less than a threshold signal based on the comparative signal from each comparator relating to the still image, it is possible to select the image signal of the synthesized still image. In all other cases, according to the same structure, it is possible to select the image signal of a still image possessing a luminance signal less than or equal to the threshold signal. Fig. 20 shows a structural example of an image communication system in which the image synthesizing apparatus according to the aforementioned Embodiment 10 is utilized. In Fig. 20, two image communication terminal units 101a, 101b are connected via communication line 102 in a manner such that interactive communication is possible (bidirectional communication). Image communication terminal units 101a, 101b comprise, respectively, video image input apparatuses 103a, 103b; still image memories 104a, 104b; image synthesizing apparatuses 105a, 105b: and display apparatuses 106a, 106b. Video image input apparatuses 103a, 103b respectively comprise a high resolution camera which can capture document text and the like in fine detail. In the following, the actions when constructing a shared drawing (image) by joint use of the still image inputted to image communication terminal unit 101a will be explained. Initially, in image communication terminal unit 101a, in order to jointly use a document by means of image communication terminal units 101a, 101b, the document provided on a desk is captured using video image input apparatus 103a. One frame of the video image is then preserved in still image memory 104a as a still image (e.g., image 108 in Fig. 21). Subsequently, image communication apparatus 107a transmits the still image preserved in still image memory 104a via communication line 102. Image communication terminal unit 101b stores the still image from image communication terminal unit 101a in still image memory 104b. Subsequently, in image communication terminal unit 101a, the video image (e.g., image 109a of Fig. 21) obtained by capturing a desktop using video image input apparatus 103a, is transmitted to image communication terminal unit 101b via communication line 102 by means of image communication apparatus 107a. In addition, image synthesizing apparatus 105a conducts synthesis such that image preserved in still image memory 104a (see image 108 of Fig. 21); the resultant video image (e.g., image 109a of Fig. 21) from video image input apparatus 103a; and the video image (e.g., image 109b of Fig. 21) from image communication terminal unit 101b, supplied via image communication apparatus 107a, appear transparent and discernible. An image 110, as shown in Fig. 21, is then supplied by display apparatus 106a. In the same manner, in image communication terminal unit 101b, the video image obtained by capturing a desktop using video image input apparatus 103b, is transmitted to image communication terminal unit 101a via communication line 102 by means of image communication apparatus 107b. In addition, image synthesizing apparatus 105b conducts synthesis such that image preserved in still image memory 104b; the resultant video image from video image input apparatus 103b; and the video image from image communication terminal unit 101a, supplied via image communication apparatus 107b, appear transparent and discernible. The resultant image is then supplied to display apparatus 106b. As explained in the aforementioned, it is possible to commonly use a document image or the like as a still image of a high precision by means of operating the above image communication system. In addition, it is also possible for a plurality of users to simultaneously line up and point to this still image, as well as make annotations using a pen. According to the aforementioned Embodiment 10, when synthesizing video images 109a, 109b and still image 108, due to change of the synthesis process according to the luminance of each pixel, reduction of the contrast can be prevented, and synthesized images can be viewed clearly. Embodiment 11 In the following, the Embodiment 11 of the present invention will be explained. Fig. 22 is a block diagram showing the structure of an image synthesizing apparatus according to the Embodiment 11. In Fig. 22, the aforementioned image synthesizing apparatus comprises an M number (M is a natural number) of input terminals 111 1 -111 M ; an N number of input terminals 112 1 -112 N , and 113 1 -113 N (N is a natural number greater than 1); an M number of luminance signal separators 114 1 -114 M ; an N number of luminance signal separators 115 1 -115 N ; comparative computing units 116 and 117; image switches 118 and 119; and output terminal 120. An M number of still images S 1 -S M are inputted, respectively, from input terminals 111 1 -111 M , and are supplied to the corresponding luminance signal separator 114 1 -114 M . Each luminance signal separator 114 1 -114 M supplies an inputted still image S 1 -S M to image switch 118, and at the same time, separates luminance signal I S1 -I SM from the respective still image S 1 -S M . and supplies these luminance signals to comparative computing unit 116. Comparative computing unit 116 compares the levels of luminance signals I S1 -I SM and reports the luminance signal of the lowest level to image switch 118. Image switch 118 then supplies the image possessing this reported luminance signal to image switch 119 as output still image S. An N number of video images V 1 -V N are inputted, respectively, from input terminals 112 1 -112 N , and are supplied to the corresponding luminance signal separator 115 1 -115 N . Each luminance signal separator 115 1 -115 N supplies an inputted video image V 1 -V N to image switch 119, and at the same time, separates luminance signal I V1 -I VN from the respective video image V 1 -V N , and supplies these luminance signals to comparative computing unit 117. An N number of threshold signals S TV1 -S TVN are respectively inputted from input terminals 113 1 -113 N , and supplied to comparative computing unit 117. Threshold signals S TV1 -S TVN correspond to luminance thresholds T V1 -T VN previously set at each video image V 1 -V N . Comparative computing unit 117 compares each luminance signal I V1 -I VN with the corresponding threshold signal S TV1 -S TVN , and then supplies a natural number k corresponding to the luminance signal with the lowest luminance from among all luminance signals I Vk which are less than or equal to the corresponding threshold signal S TVk (1≦k≦N) to image switch 119. In the case when the luminance signals I V1 -I VN of all N video images V 1 -V N are greater than their corresponding threshold signals S TV1 -S TVN , this information is reported to image switch 110. In tho case when such information 1a reported, image switch 119 outputs the inputted synthesized still image S to output terminal 120 as output image P: in all other cases, image switch 119 outputs the video image V k corresponding to natural number k to output terminal 120 as output image P. Embodiment 12 In the following, the Embodiment 12 of the present invention will be explained. Fig. 23 is a block diagram showing the structure of an image synthesizing apparatus according to the Embodiment 12. In Fig. 23, the aforementioned image synthesizing apparatus comprises an M number (M is a natural number) of input terminals 121 1 -121 M ; an N number of input terminals 122 1 -122 N (N is a natural number greater than 1); input terminal 123: an M number of luminance signal separators 124 1 -124 M ; an N number of luminance signal separators 125 1 -125 N ; comparative computing units 126 and 127; image switches 128 and 129; and output terminal 130. An M number of still images S 1 -S M are inputted, respectively, from input terminals 121 1 -121 M , and are supplied to the corresponding luminance signal separator 124 1 -124 M . Each luminance signal separator 124 1 -124 M supplies an inputted still image S 1 -S M to image switch 128, and at the same time, separates luminance signal I S1 -I SM from the respective still image S 1 -S M , and supplies these luminance signals to comparative computing unit 126. Comparative computing unit 126 compares the levels of luminance signals I S1 -I SM and reports the luminance signal of the lowest level to image switch 128. Image switch 128 then supplies the image possessing this reported luminance signal to image switch 129 as output still image S. An N number of video images V 1 -V N are inputted, respectively, from input terminals 122 1 -122 N , and are supplied to the corresponding luminance signal separator 125 1 -125 N . Each luminance signal separator 125 1 -125 N supplies an inputted video image V 1 -V N to image switch 129, and at the same time, separates luminance signal I V1 -I VN from the respective video image V 1 -V N , and supplies these luminance signals to comparative computing unit 127. A threshold signal S TV is inputted from input terminal 123, and supplied to comparative computing unit 127. Threshold signal S TV corresponds to the luminance threshold T V of a pixel commonly preset for all video images V 1 -V N . Comparative computing unit 127 compares each luminance signal I V1 -I VN , and then reports a natural number k corresponding to the luminance signal with the lowest luminance from among all luminance signals I Vk (1≦k≦N) to image switch 129. In addition, comparative computing unit 127 compares the luminance signal with the lowest luminance I Vk with the threshold signal S TV . In the case when the luminance signal I Vk is greater than threshold signals S TV , this information is reported to image switch 129. In the case when such information is reported, i.e., information that luminance signal I Vk is greater than threshold signals S TV , image switch 129 outputs the inputted synthesized still image S to output terminal 130 as output image P; in all other cases, image switch 129 outputs the video image V k corresponding to natural number k to output terminal 130 as output image P.

Claims: A method of synthesizing an image from a plurality of N input images P 1 -P N , the method comprising the steps of: comparing the luminance I p1xy -I pNxy of each pixel P 1xy -P Nxy positioned at the respective same coordinates x,y of each input image P 1 -P N with a predetermined threshold value T 1 -T N which is respectively given for each input image (SB4); determining the luminance of a synthesized pixel p xy at said coordinate x,y of a synthesized image by synthesizing said pixels p 1xy -p Nxy with a predetermined synthesis ratio given for each of said input images P 1 -P N (SB11), if the luminance I p1xy -I pNxy of all said pixels p 1xy -p Nxy is greater than the corresponding threshold T 1 -T N (SB10), or determining the luminance of said synthesized pixel p xY at said coordinate x,y by synthesizing only those pixels whose luminance is less than or equal to the corresponding threshold T 1 -T N among said pixels p 1xy -p Nxy (SB14-SB16), if the condition described above is not satisfied (SB10); and carrying out the above procedure for each image region which needs to be synthesized (SB20- SB23). A method of synthesizing an image from N video images and M still images, N being a natural number greater than 1 and M being a natural number, the method comprising the steps of: if M is 1 (SE1), taking the sole still image S 1 as a synthesized still image S (SE25); if M is greater than 1 (SE1), comparing the luminance I s1xy -I sMxy of each pixel S 1xy -S Mxy positioned at the respective same coordinates x,y of the M still images S 1 -S M with a predetermined threshold value T s1 -T sM which is respectively given for each still image (SE5); determining a synthesized still pixel s xy at said coordinate x,y of a synthesized still image S; and carrying out the above procedure for each image region which needs to be synthesized (SE21-SE24); comparing the luminance I v1xy -I vNxy of each pixel v 1xy -v Nxy positioned at the respective same coordinates x,y of the N video images V 1 -V N with a predetermined threshold value T v1 -T vN which is respectively given for each video image (SE29); taking said synthesized still pixel s xy at said coordinate x,y of said synthesized still image S as a synthesized pixel p xy at said coordinate x,y of a synthesized image P (SE37), if the luminance I v1xy -I vNxy of all said pixels v 1xy -v Nxy is greater than the corresponding threshold T v1 -T vN (SE35), or determining the luminance of said synthesized pixel p xy at said coordinate x,y of said synthesized image P among those pixels v 1xy -v nxy which have a luminance less than or equal to the corresponding threshold T v1 -T vN (SE36-SE38), if the condition described above is not satisfied (SE35) ; and carrying out above procedure for each image region which needs to be synthesized (SE40-SE43). A method according to claim 2, wherein determining a synthesized still pixels at said coordinate x,y of a synthesized still image is done by synthesizing said pixels s 1xy -s Mxy with predetermined synthesis ratio for each of said still images S 1 -S M (SE12) if said luminance I s1xy -I sMxy of said pixels s 1xy -s Mxy are all greater than the corresponding threshold T s1 -T sM (SE11) or determining said synthesized still pixel s xy at said coordinate x,y of said synthesized still image S by synthesizing with arbitrary synthesis ratio only the pixels having luminance less than or equal to the corresponding threshold T s1 -T sM among said pixels s 1xy -s Mxy (SE16,SE17) if the condition described above is not satisfied (SE11). A method according to claim 2, wherein a pixel having the lowest luminance among said pixels s 1xy -s Mxy is taken as the synthesized still pixel s xy at said coordinate x,y of a synthesized still image S (SG10). A method according to claim 4, comprising the steps of: after having compared each luminance I v1xy -I vNxy of each pixel v 1xy -v Nxy positioned at the same coordinates x,y of N video images V 1 -V N ; selecting a pixel having the lowest luminance among said pixels v 1xy -v Nxy (SG31-SG35); taking the synthesized still pixel s xy at said coordinate x,y of said synthesized still image S as a synthesized pixel p xy at said coordinate x,y of a synthesized image P if luminance of the selected pixel is greater than a predetermined threshold (SG36), or taking said selected pixel as said synthesized pixel p xy at said coordinate x,y of said synthesized image P if the conditions described above is not satisfied ; and carrying out above procedure for each region which needs to be synthesized. An apparatus for synthesizing an image from a plurality of N input images P 1 -P N , comprising: comparing means (24) for comparing the luminance I p1xy- I pNxy of each pixel P 1xy- P Nxy positioned at the respective same coordinates x, y of each input image P 1 -P N with a predetermined threshold value T 1- T N which is respectively given for each input image (SB4); synthesizing means (25, 26, 27) for determining the luminance of a synthesized pixel p xy at said coordinate x, y of a synthesized image by synthesizing said pixels p 1xy -p Nxy with a predetermined synthesis ratio given for each of said input images P 1 -P N (SB11), if the luminance I p1xy- I pNxy of all said pixels p 1xy -p Nxy is greater than the corresponding threshold T 1 -T N (SB10), or determining the luminance of said synthesized pixel p xy at said coordinate x, y by synthesizing only those pixels whose luminance is less than or equal to the corresponding threshold T 1 -T N among said pixels p 1xy -p Nxy (SB14-SB16), if the condition described above is not satisfied (SB10); and means for carrying out the above procedure for each image region which needs to be synthesized (SB20-SB23). An apparatus for synthesizing an image from N video images and M still images, N being a natural number greater than 1 and M being a natural number, comprising: means for taking the sole still image S1 as a synthesized still image S (SE25) if M is 1 (SE1) ; first comparison means (77) for comparing the luminance I s1xy -I sMxy of each pixel S 1xy -S Mxy positioned at the respective same coordinates x, y of the M still images S 1 -S M with a predetermined threshold value T s1 -T sM which is respectively given for each still image (SE5), if M is greater than 1 (SE1); first synthesizing means (79, 81, 83) for determining a synthesized still pixel s xy at said coordinate x, y of a synthesized still image S; means for carrying out the above procedure for each image region which needs to be synthesized (SE21-SE24); second comparison means (77) for comparing the luminance I v1xy -I vNxy of each pixel v 1xy -v Nxy positioned at the respective same coordinates x, y of the N video images V 1 -V N with a predetermined threshold value T v1 -T vN which is respectively given for each video image (SE29); second synthesizing means (80, 82, 84) for taking said synthesized still pixel s xy at said coordinate x, y of said synthesized still image S as a synthesized pixel p xy at said coordinate x, y of a synthesized image P (SE37), if the luminance I v1xy -I vNxy of all said pixels v 1xy -v Nxy is greater than the corresponding threshold T v1 -T vN (SE35), or determining the luminance of said synthesized pixel p xy at said coordinate x, y of said synthesized image P among those pixels v 1xy -v Nxy which have a luminance less than or equal to the corresponding threshold T v1 -T vN (SE36-SE38), if the condition described above is not satisfied (SE35); and means for carrying out the above procedure for each image region which needs to be synthesized (SE40-43). An apparatus according to claim 7, wherein said first comparison means (77) outputs first information if said luminance I s1xy -I sMxy are all greater than the corresponding threshold value T s1 -T sM , or outputs second information regarding the luminance signals less than or equal to the corresponding threshold value T s1 -T sM if the condition described above is not satisfied; said first synthesizing means (79, 81, 83) synthesizes said synthesized still image from said still images S 1 -S M with predetermined synthesis ratio for each of said still images S 1 -S M if said first information is inputted, or synthesizes said synthesized still image from said corresponding images to said second information with arbitrary ratio if said second information are inputted; said second comparison means (77) outputs third information indicating that the corresponding condition is satisfied if said luminance I v1xy -I vNxy are all greater than the corresponding threshold luminance T v1 -T vN , or outputs fourth information regarding the luminance signals less than or equal to the corresponding threshold luminance T v1 -T vN if the condition described above is not satisfied; and said second synthesizing means (80, 82, 84) outputs said synthesized still image as a synthesized image if said third information is inputted, and outputs synthesized image from image having the corresponding luminance signals to said fourth information among said images V 1 -V N with arbitrary ratio if the condition described above is not satisfied.

Inventors: ARITA KAZUHO, SATOSHI ISHIBASHI, SHIGERU OIKAWA, YAGI TAKASHI, ARITA, KAZUHO, SATOSHI, ISHIBASHI, SHIGERU, OIKAWA, YAGI, TAKASHI
Applicants: NIPPON TELEGRAPH & TELEPHONE, NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1994
Main classification: G06T5
IPC codes: G06T5